Tone-control means



July 3, 1928.- 1,675,831

R. c. sPRAGuE TONE CONTROL MEANS Filed May 29, 1925 Patented July 3,1928.

UNITED STATES ROBERT C. SPBAGUE, OF QUINCY, LIASSACHUSETTS.

'ronn-conrnon MEANS.

Application filed an as, 1925. Serial No. 33,724.

My present invention relates. to soundreproducing apparatus, particularly electrlcally operated sound reproducing apparatus, more particularly radio signalling apparatus, and specifically to a tone control means for audio frequency currents.

In radio signaling apparatus in use at the present day, and particularly that class of radio signaling apparatus known as radio telephone receiving sets, no effective way has been discovered of suitably varying the tone of the receiving instruments as telephone receiver's. or loud speakers. By means of my present apparatus I am able to P051- 1 tively and accurately vary the tone of the telephone receivers or loud speakers, and as this is the first means of which I am aware for producing this desired result, I desire to ably a variable condenser, across or in shunt with the telephone receiver'or loud speaker and, by varying the capacity of the condenser, am enabled to control the tone of the telephone receivers or loud speakers. When once the desired tone has been produced I can, by measuring the capacity of the condenser, substitute for the variable condenser a fixed condenser of that known capacity. In this connection, I am aware-that it is a common expedient to use a condenser in shunt across the telephone receivers or a radio receiving set, but my invention is to be differentiated therefrom as such condens ers as have heretofore been used are not used for the purpose for which my'apparatus is used, nor could they be used for such purpose, having ordinarily too small a capacity,v p

In the accompanying drawings illustrating a preferred embodiment of my lnvention,

Fig. 1 shows a non-regenerative detecton circuit employing a three element audion tube, and a Fig. 2 shows a modified form of Flg. 1 and to which has been added a single stage of audio frequency amplification.

Referring to the drawings, and part1c u larly to Fig. 1 wherein is shown a sin le circuit non-regenerative detector circuit employing a three element audion tube, 10 designates an audion tube, the plate 11 0f which is connected to a B battery '12 and to a pa r of telephone receivers 13 and thence by conductor 14: to he fi am n 1 The smut from plate 11 through B-battery.'12, tele phone receivers 13, conductor 14 and filament 15 is commonly known as the plate circuit. The current in the plate circuit is,

phone winding itself, through which the radio frequency component ofcurren't may bypass the high inductance of the tele hones 13; but it is sometimes necessary, an often advantageous to introduce between the points 16 and 17 an additional capacity 18 for this purpose; namely, to provide a means, so. that the radio frequency compo nent of current may bypass the high inductance of the telephones 13. This capacity is usually in the form ofa small condenser,

and, as shown, is shunted across the tele phones 13. The use of such condenser as 18 has been known for a long time and it is ordinarily called the phone condenser. In use, it has a customary value of from 0.002- 0.004 of a microfarad. While the condenser 18 could be made variable, this is not the usual custom. As above stated, it is possible to operate this circuit without the use of a condenser 18, and this is true particularly if the'telephone leads from the battery 12 and filament 15 to the telephones are of considerable length. By the use of the con denser 18, however, the radio frequency component of the current in the plate circuit above referred to is by-passed and practically none of such component passes into .the conductors leading to the telephones 13.

Therefore, the only component of the current in such telephone 13, or in the direct circuit leading thereto, is the audio frequency component. This being so, it will be obvious that, no means having been s ecially provided for affecting the audio requency component, the use of the condenser 18 can haveno effect whatever on the tone produced on the telephone receivers 13 by such audio frequency component.

Referring now to Fig. 2 wherein is shown the simple receiving circuit of Fig. 1 with the addition thereto of a single sta e of audio frequency, that portionof 1g. 2

w th t e'lik e ement in Fig. 1 ha e en where the elements therein are identical given the same reference characters. From the reasoning as applied to Fig. 1 it is evident, therefore, that, if the telephone 13 has substituted therefor, as it has in Fig. 2, the primary .winding 19 of an audio frequency transformer, the only-component of the current in the plate circuit passing through such primary will be the audio frequency component. The best audio frequency transformers are only designed to pass on audio frequency impulses between the range of 50-10,000 cycles per second. The radio frequency impulses, which are of the order of- 1,000,000 cycles per second, being choked out of the amplifying circuit and b -passed through the separate condenser in ig. 2 or, if the circuit is properly designed, the condenser 18 in Fig. 2 may be dispensed with and dependence placed on the capacity of the primary winding 19. The secondary Winding 20 of the transformer is in circuit with thethree element audio tube 21 and into the plate circuit thereof, such plate circuit consisting of a late 22, B battery 23, loud speaker 24, con uctor 25, and the filament 26. The only current in the plate circuit last described is the alternating current of audio frequency and, therefore, there is points 16 and 17 ,in either Fig. 1 or Fig. 2 to no need of a by-pass condenser between the points 27 and 28 of such circuit as existed for the by-pass condenser 18 between the insure proper electrical operation of t set. Perhaps it is for this reason that, heretofore no condenser, either fixed or variable, has been found necessary in the situation just referred to for, as stated above, the use of the condenser is a by-pass condenser to pass the radio fre uency component of current around the te ephone receivers and allow only the audio frequency component to pass through such receivers. I have found, however, that if a variable condenser of proper capacity, as the condenser 29, is shunted across the loud speaker or telephones 24, at the points 27 and 28, a control is obtained over the tone of the telephone receiver or loud speaker 24, and by experimentation I have found that the capacity of the condenser 29 should vary approximately from 0.01-0.05 of a microfarad. The use of the tone control condenser in all cases has greatly improved the quality of the tone of the instrument and thus in device is capable of having the tone 0 the instrument placed under control of the operator. This phenomenon of tone control is practically due to the carrying over and broadening of the tones as the tone control condenser changes the phase angle of the audio frequency current that passes through the loud speaker or telephones 24.

The capacity of a tone control condenser is approximately 0.01-0.05 of a microfarad. The phone condenser is used as a radio frequency current by-pass. The tone control condenser is used to control the tone of the receiving instrument.

Having thus described my invention, what I claim as new is:

1. An improved tone control for radio receiving circuits, comprising an output circuit, means for delivering audio frequency current thereto, telephone receivinginstruments in series in said circuit, and a variable condenser of from 0.01 to 0.05 of a microfarad capacity in shunt to said receiving instruments.

2. The improved method of controlling the tone of the translatin instruments of radio receiving circuits, which consists in delivering to said instruments only alternating currents of audio frequency, by-passing a portion of said currents through a variable capacity of approximately from 0.01 to 0.05 of a microfarad connected in shunt to said instruments and varying said ca acity within the above limits to obtain t e tone desired.

In testimony whereof, I have signed my name to this specification. I

ROBERT C. SPRAGUE. 

